1. Field of the Invention
The present invention relates to a method for producing a dimethylolcarboxylic acid such as dimethylolbutanoic acid which is useful as a material for producing an aqueous polyurethane resin, a polyester resin, an alkyd resin, a polycarbonate resin, a plasticizer, a lubricating oil, a surfactant, a base material for cosmetics, a reactive monomer, etc.
2. Description of the Prior Art
As a general method for producing dimethylolbutanoic acid, known is a direct oxidation method of the methylol group of trimethylolpropane (hereinafter referred to as “TMP”). The oxidation method includes chromium acid oxidation (Jones oxidation), oxidation using aluminum alkoxide (Openauer oxidation), oxidation using a noble metal such as Pt and Pd as disclosed in Japanese Patent Application Laid-Open No. 6-192169, etc.
In another method disclosed in U.S. Pat. No. 6,072,082, dimethylolbutanoic acid is obtained through the steps of producing 2,2-dimethylolbutanal, producing dimethylolbutanoic acid, and recovering dimethylolbutanoic acid. In the production process of 2,2-dimethylolbutanal, n-butylaldehyde (herein after referred to as “NBAL”) and formaldehyde are subjected to aldol condensation in the presence of a basic catalyst to produce 2,2-dimethylolbutanal. In the production process of dimethylolbutanoic acid, 2,2-dimethylolbutanal thus obtained is oxidized by hydrogen peroxide to dimethylolbutanoic acid. In the recovery step, dimethylolbutanoic acid is generally collected through the steps of neutralizing a salt of dimethylolbutanoic acid thus obtained by acid, extracting the liberated dimethylolbutanoic acid by ketone, and crystallizing dimethylolbutanoic acid.
The direct oxidation such as Jones oxidation and Oppenauer oxidation is excellent in oxidation efficiency, but not suitable as the industrial method because it requires a harmful compound, it by-produces a harmful waste, and it complicates the reaction steps. The oxidation by noble metal proceeds slowly under relatively mild conditions, and it requires a step for recovering the carboxylic acid by acid precipitation because the oxidation should be carried out under basic conditions to form a salt of the objective dimethylolbutanoic acid. In addition, since wastes such as sodium sulfate are produced, this oxidation method is not industrially practicable.
TMP has three chemically equivalent methylol groups. If TMP is freely oxidized, the by-production of a dicarboxylic acid and a tricarboxylic acid in addition to the objective monocarboxylic acid is unavoidable. To avoid this problem, a method for oxidizing TMP in a low conversion is known. This method, however, is disadvantageous in view of production efficiency and requires an additional step for removing non-reacted TMP, by-produced dicarboxylic acid, etc.
In the method by the oxidation of an alkanal, NBAL and formaldehyde are subjected to aldol condensation to produce 2,2-dimethylolbutanal. However, 2,2-dimethylolbutanal is thermally instable to reduced the yield based on NBAL. In addition, since the aldol condensation requires a basic catalyst such as sodium hydroxide, dimethylolbutanoic acid should be precipitated from a solution by acid during its purification. This makes the method industrially disadvantageous.